The present disclosure relates to a waveguide-type optical diffraction grating and an optical wavelength filter which perform wavelength selection without depending on polarization.
In recent years, as for a subscriber optical access system, a passive optical network (PON) communication system has become mainstream in which one office-side optical line termination apparatus (optical line terminal (OLT)) and a plurality of subscriber-side optical line termination apparatus (optical network units (ONUs)) are connected via an optical fiber and a star coupler, and the OLT is shared by the plurality of ONUs. In the communication system, an optical signal wavelength used in a downlink communication and an optical signal wavelength used in an uplink communication are made different so that the downlink communication directed to the ONU from the OLT and the uplink communication directed to the OLT from the ONU may not interfere with each other.
Therefore, a multiplexing/de-multiplexing device is necessary for de-multiplexing/multiplexing optical signals which are used in each of the downlink communication and the uplink communication and have different wavelengths from each other. Generally, an OLT and an ONU are constituted by spatially coupling an optical wavelength filter as the multiplexing/de-multiplexing device, a photodiode (PD) and a laser diode (LD) for realizing a function to transmit/receive optical signals having different wavelengths from each other. Although an alignment work for aligning optical axes among the optical wavelength filter, the PD and the LD becomes necessary for spatial coupling, an optical wavelength filter which makes the alignment work for aligning optical axes unnecessary and are constituted by using a waveguide, has been developed. In addition, a technology using silicon-based material as waveguide material attracts attention because of its superiority in miniaturization and mass productivity in forming the optical wavelength filter. (For example, refer to U.S. Pat. No. 4,860,294B, U.S. Pat. No. 5,764,826B, U.S. Pat. No. 5,960,135B, U.S. Pat. No. 7,072,541B and JP H08-163028A).
As an optical wavelength filter which can be used in the subscriber optical access system, an optical wavelength filter which uses a Mach-Zehnder interferometer, an optical wavelength filter which uses a directional optical coupler, an optical wavelength filter which uses a waveguide-type optical diffraction grating and the like are known. As for the optical wavelength filter which uses the Mach-Zehnder interferometer made up of a silicon waveguide, it is difficult to miniaturize the device because it is necessary to connect Mach-Zehnder interferometers in multiple stages. In addition, when the directional optical coupler is used as an optical wavelength filter, it is vulnerable to a wavelength deviation of a light source because a transmission factor within a transmitting region has wavelength dependency. In addition, as for the directional optical coupler, it is also difficult to miniaturize the device because its device length is approximately hundreds of micrometers.
As an optical wavelength filter which uses the waveguide-type optical diffraction grating, an optical wavelength filter which is formed by using a silicon substrate is disclosed (refer to Hirohito Yamada, et. al., “Si Photonic Wire Waveguide Devices” IEICE Transactions of Electronics vol. E90-C, No. 1, pp. 59-64, January 2007). In addition, an optical wavelength filter which uses the waveguide-type optical diffraction grating of a mode conversion type in a multimode waveguide is also disclosed in JP 2006-235380A. Alternatively, an optical wavelength filter constituted by combining the waveguide-type optical diffraction grating and the directional optical coupler is also disclosed (refer to Wei Shi, et. al., “Add-Drop Filters in Silicon Grating-Assisted Asymmetric Couplers” Optical Fiber Communication Conference 2012 OTh3D.3).
Furthermore, disclosed is a device with diversity in the reflection spectrum characteristics while various deformed regions are formed in a periodic structure of the waveguide-type optical diffraction grating. For example, a structure where ¼ wavelength shift (λ/4 phase shift) region is provided in the middle of the waveguide-type optical diffraction grating is disclosed in JP 2003-309321A, and an optical wavelength filter having a structure where a period of the waveguide-type optical diffraction grating changes gradually is disclosed in JP H11-14858A. According to a configuration, a transmitted wavelength peak having a very thin line width is acquired in a transmitted light spectrum.
In addition, a device in which a modulated period diffraction grating (sampled grating) is adopted as the waveguide-type optical diffraction grating is disclosed (refer to JP 2013-258398A and JP 2013-16650A). The optical wavelength filter based on the sampled grating has a plurality of transmitted wavelength peaks in the transmitted light spectrum, and therefore, is suitable to be used as a wavelength separation filter for a multi-channel optical signal based on frequency multiplexing.